As a rare and aggressive neurological disorder, cystinosis affects only 500 people living in the United States today. This disorder originates from a recessive gene called CTNS, which is responsible for managing the deposition of cystine crystals in the cornea, a light-bending layer of the eye. This gene is able to regulate a required level of cystine through the creation of a protein called cystinosin. Cystine is an amino acid that is vital for making important proteins in our body. In cystinosis, cystine is accumulated in excess amounts, and frequently affects the eyes in a detrimental manner. When cystine builds up in the corneal layer of the eye, it may give an appearance of glass shards after forming into crystals.
This figure demonstrates how a mutated CTNS gene can impact the pathway of cystine production. When the CTNS gene is mutated, the generation of the protein cystinosin, and subsequently cystine, are altered.
Cystinosis is developed when someone receives a mutated copy of the CTNS gene from each parent. Due to this pathway requiring both parents to have this abnormal gene, cystinosis is an incredibly rare disorder.
As an autosomal recessive disease, cystinosis must be passed down to a child by two parents who are carriers or have the disorder themselves. In this image, two parents that are "carriers" of the mutated CTNS gene have a 25% chance of having a child with cystinosis.
While it is not a commonly occurring condition, cystinosis causes multiple unpleasant symptoms that mainly affect the cornea of the eye. The cornea’s primary functions are to alter the path of light as it enters the eye, as well as protect the inner eye from damage and pathogens. Cystine accumulates in the front of the cornea first, and as the disease progresses, it works its way inward. When cystine develops deeper in the cornea, this layer loses the protective and light-altering properties it has in a healthy eye.
The presence of cystine crystal accumulation in the cornea.
A patient with cystinosis may experience deteriorating vision, eye pain, and having difficulty completing daily activities. The most notable symptom with cystinosis, however, is photophobia. Photophobia is associated with light sensitivity, and may cause someone to have discomfort when looking at a bright light. This manifestation is caused by the impairment of the cornea, which normally has the ability to regulate the light entering the eye. On top of these physically painful indicators, an eye exam can reveal a visualization of these crystal depositions in the cornea. While the cornea is deeply impacted by cystinosis, the retina is affected negatively as well.
In the retina, which has a normal function to send electrical signals to the optic nerve and process our vision, cystinosis promotes the build-up of cystine crystals along this layer. Accumulation in this layer of the eye can lead to peripheral pigmentary changes, which indicate death of the photoreceptors located in the retina. When these photoreceptors are damaged, irreversible vision loss is likely to occur.
Cone and rod cells are photoreceptors that reside in the retina and function to transport the electrical activity that our eyes receive. These may be irreversibly destroyed in cases of retinal cystinosis.
Treatment for cystinosis is a research subject that is continuing to develop. A few developed therapies for cystinosis include Cystine Depleting Therapy and Symptomatic Therapy. To discover more about the mechanisms behind each of these treatments, take a look at this article by the National Organization for Rare Disorders:
interesting - I hadn’t heard about this before.
In the last paragraph did you mean renal? You hadn’t mentioned anything about kidney effects.